Stents and stent delivery devices are employed in a number of medical procedures and as such their structure and function are well known. Stents are typically cylindrical, radially expandable prostheses introduced via a catheter assembly into a lumen of a body vessel in a configuration having a generally reduced diameter, i.e. in a crimped or unexpanded state, and are then expanded to the diameter of the vessel. In their expanded state, stents support or reinforce sections of vessel walls, for example a blood vessel, which have collapsed, are partially occluded, blocked, weakened, or dilated, and maintain them in an open unobstructed state. They have also been implanted in the urinary tract and in bile ducts. Stents are advantageous because they can reduce the likelihood of restenosis, and because the patency of the blood vessel is maintained immediately after the intravascular treatment.
Stents are available in self-expanding and in inflation expandable versions which employ a dilatation balloon for expansion. Both versions are well known and widely available in a variety of designs and configurations, and are made from a variety of materials including plastics and metals with metals being more typically used.
For stent delivery, the stent and optionally a balloon, are positioned at the distal end of the catheter, around a core lumen. The stent and balloon are held down and covered by a sheath or sleeve. When the distal portion is in its desired location of the targeted vessel the sheath or sleeve is retracted to expose the stent. Self-expanding stents are typically in unexpanded state using a variety of methods including sheaths or sleeves, for example, during delivery to the site of deployment.
The catheter delivery system is advanced through the patients vascular system until the stent is at the desired location within a blood vessel, and then the expandable member is inflated on the catheter to expand the stent within the blood vessel. For the self-expanding variety, the sheath or sleeves may be retracted, leaving the stent free to self-expand. For the inflation variety, the expandable member, i.e. the balloon, may be expanded using a variety of methods such as through an inflation fluid, once the sheath or sleeves are removed, and the stent expands with the expandable member. The expandable member is then deflated and the catheter withdrawn, leaving the expanded stent within the blood vessel to hold the occluded or blocked vessel open.
In any case, the advancement of the stent through a patient's vasculature can involve traversing sharp bends and other obstacles which may require the stent to be highly flexible. Stent flexibility also permits the stent to be deployed in and conform to a tortuous section of a patient's vasculature.
It has also become highly desirable and common to employ a visualization technique referred to as radiographic fluoroscopy and magnetic resonance imaging (MRI), to allow the physician to accurately position and patency the stent at the desired location, and also to check the position during follow-up visits. The techniques require the stent to have good radiographic properties for fluoroscopy and good magnetic properties that avoid stent movement, floating, and image artifact during MRI.
One property is sometimes achieved at the expense of the other. Flexibility can be improved by decreasing the thickness of the material. On the other hand, radiopacity, for stents formed from stainless steel or NITINOL®, for example, is determined by the radiographic density of the material and the thickness of the stent tubing wall. Therefore, the use of thin material can reduce the radiopacity of the stent, making it more difficult to accurately visualize the stent. Conversely, the use of thicker material which can improve radiopacity, can reduce stent flexibility which can negatively impact the deliverability of the stent in a patient's vasculature.
MRI safety and compatibility is optimal when materials having low magnetic susceptibility are employed such as tantalum, titanium, niobium, and so forth. However, such materials are not necessarily optimum for properties such as flexibility, radial strength, and/or radiopacity, for example.
One method for improving the radiopacity is to deposit a thin surface coating or plating or a more radiographically dense material such as gold. This may change the properties of the stent, however, and these highly radiographic materials are often very costly.
Other desirable properties include radial strength so that the stent is resistant to elastic recoil and radial compression. This can also be difficult to achieve with a thin walled and small diameter stent.